1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography (MRT) as employed in medicine for examining patients. The present invention is specifically directed to a magnetic resonance tomography apparatus as well as to a method for the operation thereof wherein magnetic resonance data are obtained from two immediately adjacent slices so the measurement time can be shortened without SNR (signal-to-noise ratio) loss.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized as an imaging method in medicine and biophysics for more than 15 years. In this examination method, the subject is exposed to a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject align, these having been previously irregularly oriented.
Radiofrequency energy can then excite these “ordered” nuclear spins to a specific oscillation. This oscillation generates the actual measured signal in MRT, this being detected with suitable reception coils. The measurement subject can be spatially encoded in all three spatial directions by utilizing non-homogeneous magnetic fields generated by gradient coils. The method allows a free selection of the slice to be imaged, so that tomograms of the human body can be registered in all directions. In medical diagnostics, MRT is particularly distinguished as a tomographic imaging method which is “non-invasive” as a result of a versatile contrast capability. Due to the excellent presentation of soft tissue, MRT has developed into a method that is often superior to X-ray computed tomography (CT). MRT is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality with measurement times on the order of magnitude of minutes.
The ongoing technical improvement of the components of MRT apparatuses and the introduction of fast imaging sequences have made more areas of application in medicine accessible to MRT. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples. Despite the technical advances in building MRT apparatuses, exposure time and signal-to-noise ratio (SNR) of the MRT image remain limiting factors for many applications of MRT in medical diagnostics.
One method of enhancing the relationship of SNR per measurement time is the simultaneous measurement of multiple slices. Either the measurement time can be shortened—given an SNR that remains the same—or the SNR can be enhanced—given a measurement time that remains the same.
One approach for the former—i.e. unmodified SNR and shortening the measurement time—is to reduce the quantity of image data to be acquired. In order to obtain a complete image from such a reduced dataset, either the missing data must be reconstructed with suitable algorithms or the faulty image must be corrected from the reduced data. The registration of the data in MRT occurs in k-space (frequency domain). The MRT image in the image domain is linked to the MRT data in k-space by Fourier transformation. The location coding of the subject, which defines k-space, occurs by means of gradients in all three spatial directions. A distinction is made between the slice selection (determines an exposure slice in the subject, usually the z-axis), the frequency coding (determines a direction in the slice, usually the x-axis) and the phase coding (determines the second dimension within the slice, usually the y-axis). Without limitation as to universality, a Cartesian k-space that is sampled row-by-row is assumed below. The data of an individual k-space row are frequency-coded with a gradient upon readout. Each row in k-space has a spacing Δky that is generated by a phase-coding step. Since the phase-coding takes a great deal of time compared to the other spatial coordinates, most methods for shortening the image measurement time are based on a reduction of the number of time-consuming phase-coding steps, for example partially parallel acquisition (abbreviated below as PPA). The basic idea in PPA imaging is that k-space data are not acquired by an individual coil but by, for example, a linear arrangement of component coils, a coil array. Each of the spatially independent coils of the array has associated spatial information that is utilized in order to achieve a complete location encoding via a combination of the simultaneously acquired coil data. This means that a number of other, unsampled rows shifted in k-space also can be defined from a single, registered k-space row.
The PPA methods thus employ spatial information that is contained in the components of a coil arrangement in order to partially replace the time-consuming phase encoding that is normally generated upon employment of a phase gradient. As a result, the image measurement time is reduced, corresponding to the ratio of the number of rows of the reduced dataset to the number of rows of the conventional (i.e. complete) dataset. In a typical PPA acquisition, only a fraction (½, ⅓, ¼, etc.) of the phase-coding rows are acquired compared to the conventional acquisition. A specific reconstruction is then applied to the data in order to reconstruct the missing k-space rows, and thus to obtain the full field of view (FOV) image in a fraction of the time.
Whereas some of these PPA techniques have been successfully employed in many areas of MRT-SMASH (SiMultaneous Acquisition of Spatial Harmonics) and SENSE (SENSitivity Encoding) are the most noteworthy—a significant disadvantage of these methods is that the coil profiles must clearly differ in the superimposed slices. This is only true for slices that are relatively far apart and therefore cannot be applied to immediately adjacent slices. A typical distance between the coil elements of an array coil is approximately 10 cm. A slice spacing of simultaneously encoded slices of precisely this order of magnitude arises therefrom. Moreover, a slice can generally not be freely selected but must be matched to the geometry of the reception coil array.
One approach for enhancing the SNR given an unaltered acquisition time is to simultaneously excite multiple slices with specific excitation pulses. The problem then is to reconstruct separated slices from the raw data signal.
One method with which two or more slices are simultaneously excited and subsequently reconstructed was presented in 1988 by S. P. Souza et al., Journal of Computer Assisted Tomography, 12(6):1026-1030 (1988) and by S.Müller, Magnetic Resonance in Medicine, 6:364-371 (1988). The radiofrequency excitation pulse, whose base band: signal usually represents a square-wave or sinc (si) function, is modulated with corresponding offset frequencies ω1, ω2, etc. (compared to the basic frequency). As a result the slices corresponding to ω1, ω2, etc. are excited:RF(t)=si(t)eiω1t′eiω1t′  
The excitation by RF(t) as well as the following response signal measurement are multiply implemented. The slices are encoded with different operational sign from measurement to measurement. By means of a suitable combination of signal subtraction and signal addition, the slices superimposed in the measured signals can in turn be subsequently reconstructed. This results in the SNR being improved in each superposition. A disadvantage in this method, however, is the fact that a measurement is required for each slice, so that the SNR can in fact be improved but the measurement time remains the same compared to conventional encoding.